1. Field of the Invention
The present invention relates generally to the field of optical communications, and more specifically, to routing optical signals.
2. Discussion of Related Art
Communication networks increasingly rely upon optical fiber for high-speed, lowcost transmission. Optical fibers were originally envisioned as an optical replacement for electronic transmission media, such as high-speed coaxial cable and lower-speed twisted-pair cable. However, even high-speed optical fibers are limited by the electronics at the transmitting and receiving ends. For switching purposes, operating speeds are generally rated at a few gigabits per second, although 40 Gb/s systems have been prototyped. Such high-speed electronic systems are expensive and still do not fully exploit the inherent bandwidth of fiber-optic systems, which can be measured in many terabits per second.
All-optical transmission systems offer many intrinsic advantages over systems that use electronics within any part of the principal transmission path. Wavelength division multiplexing is a commonly used technique that allows the transport of multiple optical signals, each at a slightly different wavelength, on an optical fiber. The ability to carry multiple signals on a single fiber allows that fiber to carry a tremendous amount of traffic, including data, voice, and even digital video signals. For example, the use of wavelength division multiplexing, in combination with time division multiplexing, permits a long distance telephone company to carry thousands or even millions of phone conversations on a single fiber. Wavelength division multiplexing makes it possible to effectively use the fiber at multiple wavelengths, as opposed to the costlier option of installing additional fibers.
Using wavelength division multiplexing, optical signals can be carried on separate optical channels with each channel having a wavelength within a specified bandwidth. It is advantageous to carry as many channels as possible within the bandwidth where each channel corresponds to an optical signal transmitted at a predefined wavelength. Separating and combining, or demultiplexing and multiplexing, wavelengths with such close channel spacings requires optical components that have high peak transmission at the specified wavelengths and which can provide good isolation between the separated wavelengths.
U.S. Pat. No. 4,655,547 to Heritage, et. al., entitled xe2x80x9cShaping Optical Pulses by Amplitude and Phase Masking,xe2x80x9d which is herein incorporated by reference, discloses how an input optical signal can be divided into frequency components, where each frequency component is separately phase-modulated or amplitude-modulated. The input signals can be divided into spatially separated frequency components with a diffraction grating. Then, the separated channels are independently operated upon by a segmented modulator. U.S. Pat. No. 5,132,824 to Patel et al., entitled xe2x80x9cLiquid Crystal Modulator Array,xe2x80x9d which is also herein incorporated by reference, discloses using liquid-crystal modulators to manipulate optical pulses.
Currently-available wavelength routing techniques do not achieve sufficient switching contrast between individual wavelengths on a closely-spaced grid and usually involve unacceptable crosstalk between separate channels. Therefore, there is a need for a wavelength routing device which maintains a high degree of switching contrast and is able to separate or combine large numbers of wavelengths.
According to the present invention, a wavelength division multiplexed router that switches optical signals in the optical domain is disclosed. The router includes a dispersive medium that separates an input optical signal into separate optical signals or separated components, where each component corresponds to a light beam of a unique wavelength. xe2x80x9cOptical,xe2x80x9d as used herein, is not limited to the visible spectrum but includes electromagnetic radiation capable of being carried on optical fiber. Each component of the input optical signal is focused onto a reflective surface positioned to reflect the incident component in a preselected direction. Each reflected component travels back toward the dispersive medium in its respective direction and strikes the dispersive medium, which recombines certain components into a desired number of output signals. Each output signal travels through an optical medium as a separate signal.
One embodiment of the present invention utilizes a micro-mirror array modulator, which uses an array of mirrors as reflective surfaces. In some embodiments, each mirror that acts as the reflective surface is fixed in one of a limited number of angles. For example, in order to effectuate a 1xc3x972 wavelength router, each mirror is fixed in one of two angles. The reflected components travel back to the dispersive medium that initially separated an optical signal into multiple components. The dispersive medium combines the components into a desired number of output signals where some or all of the output signals may include one or more of the components of the input signal.
Other embodiments include a polarization steering device, including at least one polarization modulator, at least one birefringent polarization beam displacer, and reflective surfaces. The polarization modulator manipulates the polarization states of the component. The birefringent polarization beam displacer directs the component along a first path or a second path depending on the polarization states defined by the polarization modulator. Light beam components traveling along the first path are reflected by surfaces positioned at different angles than components traveling along the second path. The reflected components propagate back through the dispersive medium. During the propagation through the dispersive medium, the components may be recombined with other components. The combined signals, then, can be coupled into a signal transfer medium, for example optical fiber. Therefore, a single input signal can be separated into its individual wavelength components and each of the individual components can be directed into one of a number of output signals. The output signals can be coupled out of the router. The overall effect of the router is the separation of single input signal into a preselected number of output signals traveling in the desired directions.
If the polarization steering device described above contains one birefringent polarization beam displacer, it will be generally useful with an input signal of uniform polarization state. When the input signal is arbitrarily polarized, a pre-conditioning apparatus is needed to uniformly polarize the input signal. A pre-conditioning apparatus includes a birefringent polarization beam displacer that divides the input signal into two components of orthogonal polarization states, and a half-wave waveplate that rotates one of the two components.
The embodiments of polarization steering device that include two birefringent polarization beam displacers do not need the pre-conditioning apparatus described above even when the input signal is arbitrarily polarized. One embodiment configured with birefringent polarization beam displacers includes a polarization-rotating element sandwiched between two birefringent polarization beam displacers and a reflective Wollaston prism. The polarization-rotating element may be, for example, a half-wave waveplate. A polarization steering device configured with a polarization-rotating element sandwiched between two birefringent polarization beam displacers ensures that the travel distance of all components passing through the polarization steering device is substantially equal. The substantially equal travel distance minimizes polarization mode dispersion and prevents a distortion of the output signals.